The present invention relates to pumps of the type known as piston diaphragm pumps. Such pumps include a driving fluid which moves a flexible diaphragm back and forth by operation of a reciprocating piston. The back and forth movement of the flexible diaphragm pumps a driven fluid which may be paint for a spraygun or any other suitable fluid.
Prior art piston diaphragm pumps are shown in British Pat. No. 887,774 to Charles W. Simpson and in U.S. Pat. No. 3,925,988 to Zar W. Kelley.
The present invention departs from these and other prior art piston diaphragm pumps by providing a piston diaphragm pump which includes a housing having a driving fluid chamber and a driven fluid chamber. A flexible diaphragm separates the fluid chambers from one another and is movable between a rearward position and a forward position. A reciprocating piston in the driving fluid chamber is driven by a crankshaft from a forward position to a rearward position on a suction stroke and from the rearward position to the forward position on a pressure stroke. The crankshaft is arranged in a sealed crankcase, and the pump housing is secured directly to a wall of the crankcase.
A small amount of driving fluid is pumped through the driving fluid chamber on each cycle of the piston. The crankcase for the crankshaft provides a pick-up and return reservoir for the driving fluid. A normally closed inlet valve is provided in a passage extending between the crankcase reservoir and the driving fluid chamber. A mechanical actuating device on the crankshaft opens the inlet valve when the piston is in a predetermined position on its suction stroke.
By this arrangement, the amount of fluid pumped through the driving fluid chamber on each cycle of the piston can be controlled simply by changing the point at which the inlet valve is opened and closed. Additionally, a strong spring can be provided on the inlet valve to maintain it in its normally closed position to insure that the inlet valve is closed at the beginning of the pressure stroke of the piston so that fluid is not lost from the driving fluid chamber through the inlet valve before the inlet valve closes on the pressure stroke. Still further, it is believed that this arrangement, coupled with a return spring on the diaphragm, diminishes the magnitude of the negative gauge pressure in the driving fluid chamber on the suction stroke and thereby reduces the formation of air bubbles drawn out of solution from the liquid in the driving fluid chamber on the suction stroke.